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    <meta charset="UTF-8">
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    <script>
        var num1 =parseFloat(prompt("请输入第一个数"))
       var fg =prompt("请输入运算符")
       var num2 =parseFloat(prompt("请输入第二个数"))
        switch(fg){
           case ("+"):alert(num1+num2);
           break;
           case ("-"):alert(num1-num2);
           break;
           case ("*"):alert(num1*num2);
           break;
           case ("/"):alert(num1/num2);
           break;
       }
    </script>
</body>
</html>